Telephone system



June 1, 1943. LOGAN 2,320,943

TELEPHONE SYSTEM Filed Dec. 23, 1941 REGISTER MEMO/2 M. A LOGAN A TTORNE V Patented June 1, 1943 UNITE STATES TENT OFFEQE TELEPHONE SYSTEMApplication December 23, 1941, Serial No. 424,104

Claims.

This invention relates to signaling systems and more particularly to asystem in which signal impulses comprising one or more alternatingcurrent frequencies transmitted from a distant point will be received atanother point by an electric circuit which is devoid of the usual filternetworks ordinarily provided in a circuit adapted for response toalternating current pulses.

In the ordinary alternating current pulse receiver, a band-pass filteris usually provided for each frequency in the signal, each filter havinga different mid-band frequency substantially equal to the numericalfrequency of the corresponding transmitting signal oscillator. Theoutput of each band-pass filter is connected to a suitable voltagedetector which then operates on the frequency in the signal currentpassed by the band-pass filter, said filter attenuating all otherfrequencies in the signal. Suitable responding and registering devicesare, of course, connected to the detectors and the different frequenciesof which the pulse is composed, in activating their associated severaldetectors, correspondingly cause the registering devices to be set inaccordance with the several frequencies of the pulse which, iftransmitted in the form of a suitable code, will cause the registers,when set, to designate the particular intelligence designated by thecode.

According to the present invention, the receiving circuit does notemploy band-pass filters but a much simpler and less expensive networkcomprising, for each of the frequencies in the signal current to bereceived, a pair of double-balanced modulators of the rectifier type,each of which is supplied continuously with a carrier frequencysubstantially equal to the frequency of the signal. However the carriercurrent supplied to one modulator of a pair is shifted 90 in phase fromthe same carrier current supplied to the other modulator. To indicatethe presence of the frequency in a signal, a double wound relay withmechanically linked armatures is provided, one of its windings beingconnected to the output of one of the modulators and the other connectedto the output of the other modulator. The linked armatures cause thepull on each armature to add directly to actuate the contact springs.

In my copending application Serial No. 421,615, filed Dec. 4, 1941, asystem of the above type is disclosed in which one double-balancedmodulator is used for each channel and a frequency supply source is usedtherein both as the signal and as the carrier in order to provide anidentity of frequency and phase'between the two so that a direct currentcomponent will be available at the output terminals of the modulator. Itis this component of the modulated currents that is effective to operatea direct current relay provided thereat. Such a system, however, limitsthe signaling distance since it requires negligible phase shift betweenthe signal voltage received by the detector from the transmittingstation and the voltage that drives the balanced modulator, a conditionwhich also makes it necessary to use the same frequency source both forthe transmitter and the receiver.

According to the present invention, phase requirements between thesignal and carrier voltages are entirely eliminated and, further, somefrequency deviation is also permitted between the signal frequencysupply sources at the transmitting station and those applied to thereceiving modulators. Hence the transmitting and receiving stations may,according to the present invention, be remote from each other andentirely independent sources of signal and carrier currents may be usedat the transmitting and receiving stations, respectively.

A clearer conception of the scope and purpose of the invention may beobtained from the following detailed description and appended claimstaken in connection with the following drawing in which Fig. 1 shows thepaired modulators used in the invention while Fig. 2 shows the signalsystem of my invention as comprising ,a transmitting station representedby the key-set KS with five sources of frequency currents fa fe wired tothe key-set KS in accordance with a code that permits the application ofsaid frequencies to the transmission conductors TC and RC two at a timeby the depression of the individual keys of the key-set, and a receivingstation comprising five channels MCa MCe, one for each of thefrequencies that enter into the signal currents to be transmitted, asource of carrier current supply for each channel corresponding to thenominal frequency to be passed by the channel, said frequency sourcebeing connected by a suitable network to each of the paired modulatorsso as to be displaced in phase by with respect to each other, and arelay MCRa MCRe for each channel, said relay being a double wound relaywith two armatures mechanically linked as, for example, the onedisclosed-in Patent 1,207,736 issued to O. F. Forsberg on December12,1916.

Before describing the operation of my invention, reference is directedto Fig. 1 which shows the fundamental principle of the invention. .Atthe sending end, an oscillator of frequency In feeds the transmissionline, this line having both attenuation and phase shift. The receivedvoltage from the transmission line is applied to two modulatorsconnected in parallel and each comprising an input transformer IT, anoutput transformer OT and a double-balanced rectifier bridge connectedto the output and input windings, respectively, of the input and outputtransformers. Between the central taps of the input and outputtransformers of the upper modulator is connected one winding of a relayMCR, and between the central taps of the input and output transformersof the lower modulator is connected the other winding of relay MCR, thearmatures of both windings being mechanically linked together so thatthe operation of either winding will close a common pair of contacts.Each of the modulators is supplied by a carrier frequency In (nominallythe same as 1'0) but having a phase difference of 90 with respect toeach other, introduced by the network comprising condenser ON andresistance RN connected series aiding between the high input impedanceamplifiers AIVIP of the two channels and the carrier frequency sourcef0. Of course, separate oscillators differing in phase by 90 could beconnected to each output transformer of the modulators.

If frequencies in and fc are of slightly different frequency, thecurrent in each winding of the relay MCR- primarily will consist of alow frequency alternating current. The frequency of this current will beequal to that produced by the difference frequency between the carrierand signal voltage. There will also be present higher frequency termsdue to other modulation products. These frequencies; however, are highand only a small amount'of high frequency current will flow in the coilsof the relay MCR because of the inductive reactance of its windings.

Hence the current which will be effective to operate relay MCR- will beonly that which is due to the low frequency output voltage of each ofthe modulators. In the event, however, that the signal frequency isidentical with that of the carrier frequency, that is, no frequencydeviation exists between the two, a steady direct current component willbe available in either or both windings of relay MCR--.

As noted above, carriage voltage fed to one modulator is shifted 90 inphase from the voltage supplied to the other associated modulator. Whena signal source f is applied to a pair of modulators having carriervoltages differing in phase by 90, the output frequency equal to thedifference frequency of the signal in and carrier fc will be shifted inone modulator output 90 out of phase with respect to the output of theother modulator. This is true regardless of the phase of the inputsignal source. Since one winding of relay MCR is connected to the outputof one of the modulators and the other winding of the relay is connectedto the output of the other modulator, the modulated signal voltageapplied to one winding of the relay will be 90 out of phase with themodulated signal voltage applied to the other winding, and since theimpedances of the two windings are nominally equal, the steady statecurrent in the two windings are 90 out of phase. Thus, if the current inone winding is designated as I sin of, the current in the other windingcan be designated as I cos of. The armature pull due to each winding is,however, approximately proportional to the square of the current, andsince the total armature pull is equal to the sum of the pulls exertedby the two armatures, the total steady state pull P is:

P=KI (cos wt+sin wt) =Kl =constant Returning, now, to the detaileddescription of the disposition of the various parts of the circuitapparatus and the explanation of the operation of the invention as awhole and referring to Fig. 2, the frequency sources fa fe, which may beof any suitable construction each capable of emitting, respectively, thesingle frequencies arbitrarily indicated by the subscripts a e, areconnected, at the transmitting station, on the one side to conductor RCextending to the receiving channels and, on the other side over highresistances Ra Re, to certain moving springs of the key-set KS, thestationary springs of which are all multipled and connected to conductorTC, also extending to the receiving channels.

The input side of the receiving portion of the circuit of my inventioncomprises as many channels of paired double-balanced modulators as thereare frequencies to be used for signal transmission. In the embodiment ofthe invention herein disclosed and described by way of example. it isassumed that there will be five signal frequencies, in consequence ofwhich five channels M09. M06 are provided, each comprising one pair ofdouble-balanced modulators for each of the frequencies fa fe as shown.

Each of the pair of modulators in each channel is identical inelectrical properties to the one conventionally shown in Fig. Idiscussed above, and the conductors TC and RC are multipled to the inputwindings of the input transformers ITa ITZa of each of the modulators.The center taps of the output windings of the input transformers ITa ITOand the center taps of the input windings of the output transformers0T2. OTe of the upper modulators of each channel are each connected toone winding, respectively, of relays MCPta MCRb while the center taps ofthe output windings of the input transformers ITZa ITZe and the inputwindings of the output transformers OT2a OT2 are each connected,respectively, to the other winding of said relays. Thus, the upperwinding of relay MCRa is connected to the center taps of thetransformers ITa and GT3. of the upper modulator of channel MCE, whichis adapted for response to frequency a, while the lower winding isconnected to the center taps of transformers IT2a and OTZa of the lowermodulator of this channel. In the same way, the two windings of relayMCRe are connected to the center taps of the input and outputtransformers of the two modulators of channel MCe, the connection ofcorresponding relays to the paired modulators of other channels(channels MCt, IV-[Cc and MCd being merely indicated in the drawing)being the same.

Each pair of modulators of a channel is supplied, through suitable highinput impedance amplifiers AMP, by a separate carrier oscillator thenominal frequency of which is the same as the signal frequency which thechannel is adapted to detect and, in any pair of modulators, the carriersupplied to one member is shifted in phase from the carrier in the othermember because the voltage drop across a condenser CN in series with aresistor RN lags 90 behind the voltage drop across the resistor RN-. Theregistration of any digit is then accomplished by the simultaneousoperation of two of the channel relays MCRa. MCRe.

As already stated, the frequency sources at the transmitting station arewired to the conductor RC and to the moving springs of the key-set KS inaccordance with the combination of frequencies called for by the pulseto be transmitted by the depression of the individual numerical keys 1 0of the key-set KS.

It may be assumed by way of illustration that each pulse will consist ofa combination of two frequencies which are chosen in accordance with thefollowing code and that, accordingly, the resistances Ra Re are wired tothe various numerical keys of key-set KS in the manner shown in orderthat the depression of each of the digit keys will complete two circuitsfrom the two frequency sources involved to the input side of theparallel transformers ITa IT2e of the receiving channels MCa MCe.

Pulse code Digit Frequencies transmitted Now let it be assumed thatafter the receiving channels MCa MCe have been made ready for thereception of the signals by the connection of the carrier frequencysources fa fee to the amplifiers AMP of the paired modulators of thevarious channels, digit key 8 is depressed. A circuit will then becompleted from one side of signal frequency source fa, resistance Ra,left contacts of key 8, conductor TC, the input windings of thetransformers ITa IT2e in parallel, conductor RC, to the other side ofsignal frequency source fa. Another circuit will be completed from oneside of signal frequency source je, resistance Re, right contacts of key8, thence as previously traced to conductor EC, to the other side ofsignal frequency source Re. The current composed of frequencies a and ewill be maintained over the circuit path above traced as long as key 8is kept depressed.

Since the current is composed of frequencies a and e, then, sincefrequencies a and e are connected to channels MC?. and Moe,respectively, as carriers, frequency a will enter the two modulators ofchannel MCa to produce a low frequency two phase current one phase ofwhich, available across the center taps of the windings ITa and OTa,will flow through the upper winding of relay MGR-a while the otherphase, available across the center taps of transformers ITza and OTZa,will flow through the lower winding of relay MCRa. In the same manner,frequency 6 will be impressed upon the two modulators of channel MCe,and since the frequency source fe is connected to this channel as acarrier, the output voltage across the center taps of the input andoutput transformers of its two modulators will be productive of asimilar two phase current that will flow through the windings of relayMCRe causing its operation. Pulse frequencies a and e will, of course,be impressed on the remaining channels (as well as upon channels MCa andM08 with frequency a impressed upon channel MCe and frequency 6impressed upon channel MCa) but since these channels have carrierfrequencies different from the signal frequencies, then, as abovedescribed, their output voltages will comprise currents of higherfrequency components to which the individual channel relays MCR willoffer a high impedance and will remain unaffected thereby.

The input receiving channel of my invention comprises as many channelsof paired double- This term will become a constant (in other words, adirect current component) if any frequency is equal to three times thevalue of any carrier frequency in the system, thus making the range lessthan two octaves. However, as a constant frequency difference betweensignal frequencies can be maintained for a given degree of selectivity,the lowest signal frequency may be made as high as desirable to includeany specified number of channels in a two frequency range.

Relay MGR/a and relay MORE remain operated as long as key 8 remainsdepressed, after which the signal circuit is disrupted and the relayswill release. In the same manner, the depression of other numerical keyswill cause the application of corresponding combinations of twofrequencies to the conductors TC and RC and the consequent operation ofthe two relays of the channels to which the signal frequency sources areconnected on their output side as carriers.

The operation of the channel relays MCRa MCRe, in combination of twos inresponse to a pulse of two frequencies, represents a setting of therelays that designates the intelligence to be conveyed by the pulse.And, of course, if this intelligence is to have further utility, thevarious settings of the channel relays will have to be permanentlyregistered in a suitable device the character and construction of whichand the manner of its interconnection to the channel relays Will dependupon the code in which this intelligence is to be registered in theregistering device. Since the invention is not concerned with the manneror means in which the transmitted intelligence is either registered orutilized, such means are not disclosed except schematically as aregistering circuit" responsive to the operated contacts of the channelrelays, reference being made to my above-mentioned copending applicationfor a disclosure of one type of device by which the received pulses maybe registered.

While I have described the principle of my invention in connection witha specific embodiment thereof relative to its operation with a givencode of signal pulses, it is to be understood that various otherembodiments thereof may be made by those skilled in the art withoutdeparting from the spirit of the invention as defined within the scopeof the appended claims.

What is claimed is:

1. A signal receiving circuit for an alternating current signal systemhaving a pair of rectifier networks for each signal frequencytransmitted over said system, a carrier source of current connected toeach of said rectifier networks having substantially the same frequencyas the signal frequency and displaced in phase by 90 from each other,and means connected to said pair of rectifier networks which isresponsive to the low frequency two phase components of the outputcurrent of the combined carrier and signal frequencies applied to saidpair of networks.

2. An alternating current signal system comprising a transmittingstation, a receiving network including a pair of balanced ringmodulators, a transmission medium interconnectin said transmittingstation and said receiving network, a frequency source connected tosaidtransmitting station for application as a signal to said pair ofbalanced ring modulators, a carrier frequency source connected to eachof said modulators of substantially the same frequency as the signalfrequency and displaced in phase by 90 from each other, andelectromagnetic means connected to each of said modulators which isresponsive to the low frequency two phase components of the outputcurrent of the frequencies applied to said pair of modulators ascarriers and the signal frequency applied to the said pair of modulatorsfrom the transmission medium as a signal.

3. An alternating current signal system comprising a transmittingstation, a receiving station comprising a pair of balanced rectifiernetworks, a transmission medium interconnecting said transmissionstation and said receiving station, a source of signal frequency currentconnected to said transmitting station whereby current of the frequencygenerated by said source is transmitted from said transmitting stationover said transmission medium for application to said receiving stationas a signal, a two phase current supply of substantially the samefrequency as the signal frequency connected to said pair of balancedrectifier networks, an electromechanical device having two mechanicallylinked armatures and a winding connected, respectively, to each of saidbalanced rectifier networks, each one of which winding is responsive toone of the two phases of the low frequency component of the outputcurrent produced by the frequency applied to the network as a signalfrom the transmitting station and the frequencies applied to the networkas a carrier.

4. In an alternating current signal system, the combination with atransmitting station and transmitting medium of a receiving network forsignal frequency signals transmitted from said station which comprises asource of signal current of a certain frequency at said transmittingstation, a pair of double-balanced rectifier networks, a pair ofconductors interconnecting said transmitting station and said networks,a source of current of substantially the same frequency as the signalcurrent connected to one of said double balanced rectifiers, a source ofcurrent substantially of the same frequency as the signal currentconnected to the other of said double-balanced rectifiers but displacedin phase from the source connected to said one of said pair ofdouble-balanced rectifiers, and a two winding relay having two separatearmatures mechanically interlinked, one of said windings being connectedto the output terminals of one of said pair of double-balancedrectifiers and the other of said windings being connected to the outputterminals of the other of said pair of double-balanced rectifiers,whereby each of said windings operates both armatures in response to thelow frequency components of the output current produced by themodulation of the signal current applied to each of said doublebalancedrectifiers from said transmitting station over said transmission mediumand each of the sources of current separately applied to each of saidbalanced rectifiers.

5. A signal receiving circuit for an alternating current signal systemcomprising a transmitting station, a plurality of pairs ofdoublebalanced rectifiers, one pair for each signal frequencytransmitted over said station, a transmission medium interconnectingsaid' transmitting station with said plurality of pairs ofdouble-balanced rectifiers, sources of signal frequencies connected tosaid transmitting station, the connection of said sources to saidstation being determined by the combination of signal frequencies to betransmitted by the operation of said station, a source of signalfrequency connected to each of one of said double-balanced rectifiers,the sources connected to each pair of said double-balanced rectifiersbeing substantially of the same frequency as the signal frequencyallocated to each of said pairs of doublebalanced rectifiers and shiftedin phase by 90 from each other, and a direct current responsive meansconnected to the output of each pair of said double-balanced rectifierswhich is responsive to the low frequency current components of theoutput current produced by the signal frequency transmitted from saidstation and applied to the pair of double-balanced rectifiers which areallocated to pass said frequency, and the frequencies produced by thefrequency sources connected to each of said pair of rectifiers.

MASON A. LOGAN.

